Centrifugal pumps are used for transporting fluids at a desired flow and pressure from one location to another, or in a recirculating system. Examples of such applications include, but are not limited to: oil, water or gas wells, irrigation systems, heating and cooling systems, multiple pump systems, wastewater treatment, municipal water treatment and distribution systems.
In order to protect a pump from damage or to optimize the operation of a pump, it is necessary to know and control various operating parameters of a pump. Among these are pump speed, pump torque, pump efficiency, fluid flow rate, minimum required suction head pressure, suction pressure, and discharge pressure.
Sensors are frequently used to directly measure pump operating parameters. In many applications, the placement required for the sensor or sensors is inconvenient or difficult to access and may require that the sensor(s) be exposed to a harmful environment. Also, sensors add to initial system cost and maintenance cost as well as decreasing the overall reliability of the system.
Centrifugal pumping systems are inherently nonlinear. This presents several difficulties in utilizing traditional closed-loop control algorithms, which respond only to error between the parameter value desired and the parameter value measured. Also, due to the nature of some sensors, the indication of the measured parameter suffers from a time delay, due to averaging or the like. Consequently, the non-linearity of the system response and the time lag induced by the measured values makes tuning the control loops very difficult without introducing system instability. As such, it would be advantageous to predict key pump parameters and utilize each in a feedforward control path, thereby improving controller response and stability and reducing sensed parameter time delays.
As an example, in a methane gas well, it is typically necessary to pump water off to release trapped gas from an underground formation. This process is referred to as dewatering, where water is a byproduct of the gas production. The pump is operated to control the fluid level within the well, thereby maximizing the gas production while minimizing the energy consumption and water byproduct.
As another example, in an oil well, it is desirable to reduce the fluid level above the pump to lower the pressure in the casing, thereby increasing the flow of oil into the well and allowing increased production. In practice, the fluid level is ideally reduced to the lowest level possible while still providing sufficient suction pressure at the pump inlet. The minimum required suction head pressure of a pump is a function of its design and operating point.
Typically, centrifugal pumps are used for both oil and gas production. As fluid is removed by the pump, the fluid level within the well drops until inflow from the formation surrounding the pump casing equals the amount of fluid being pumped out. It is desirable that the pump flow rate be controlled in a manner precluding the fluid level from being reduced to a point where continued flow from the well is compromised, and/or damage to the pump could occur.
If the fluid level in the well drops too low, undesirable conditions known as “pump-off,” or “gas-lock,” may occur in the pump. Pump-off occurs when the fluid level in the well has dropped to a point where the pump inlet no longer receives a steady inflow of mostly liquid fluid from the well. Gas-lock occurs, in wells having gas entrained in the fluid, when the fluid level has been reduced to such a low level that fluid pressure at the pump inlet falls below a bubble-point of the fluid, at which larger volumes of free gas are released and enter the pump. Under either a pump-off or gas-lock condition, the pumping action becomes unstable and flow is significantly reduced, with a corresponding reduction in pumping torque and motor current being observed in an electrical motor driven pump.
When a pump-off condition is encountered, it is necessary to slow down, or stop, pumping until the fluid level in the well has been sufficiently replenished, through inflow from the formation surrounding the pump casing to a level whereat the pump-off condition will not be immediately encountered upon re-starting of the pump. With a pump-off condition, it is necessary for the fluid level to rise far enough in the well that the pump inlet can once again receive sufficient inflow of mostly liquid fluid for the pump to function properly. For a gas-lock condition, it is necessary to allow the large volume of gas which caused the gas-lock condition to move upward in the tube, with a corresponding downward movement of non-gaseous fluid within the tube into the pump, so that the pump may once again function properly. Recovery from a gas-lock condition thus also requires slowing down or stopping the pump to allow for movement of gas and liquid within the tube.
As will be readily recognized, by those having skill in the art, if pumping is resumed at the pumping speed which led to either the pump-off or gas-lock condition, it is likely that such a condition would re-occur. Unfortunately, in the past, wellbore pumping systems and controls did not provide a convenient apparatus or method for determining what the maximum pump speed should be, during recovery, in order to preclude triggering a subsequent pump-off and/or gas-lock condition. In the past, motor current was sometimes monitored, and the pump was simply shut down and allowed to stand idle, for a time, whenever the value of pump current dropped below a pre-determined under-load value of current thought to be indicative of a pump-off and/or gas-lock condition. It was then necessary to let the pump remain idle, for an undetermined length of time, so that proper conditions could be re-established at the pump, by virtue of inflow of fluid to the well from the surrounding structure, and/or downward flow of non-gaseous fluid within the outlet tube connected to the pump.
Knowing when to resume pumping, and knowing what reduced pump speed should be utilized following resumption of pumping, to preclude having a recurrence of the pump-off or gas-lock condition, has been largely a matter of trial and error in the past. During the time that the pump is shut down for recovery, no revenue is being generated by the well. In addition, the uncertainty, in the past, with regard to avoiding a pump-off or gas-lock condition, and the time and procedure involved for recovering from such conditions, has led to undesirable wear and tear on the pumping equipment, as well.
It is desirable, therefore, to have an improved apparatus and method for detecting, and precluding a pump-off or gas-lock condition. It is also desirable to have an improved apparatus and method for recovering from a pump-off and/or gas-lock. It is further desirable, to have an improved apparatus and method which is capable of determining what a minimum fluid level in the well should be, in order to preclude a pump-off and/or gas-lock condition, together with a corresponding detection and control apparatus and method for determining a pump speed which will result in maintaining the fluid level at or near the desired minimum fluid level in the well.
Generally, in the past, the fluid level has been sensed with a pressure sensor inserted near the intake or suction side of the pump, typically 1000 to 5000 feet or more below the surface. These down-hole sensors are expensive and suffer very high failure rates, necessitating frequent removal of the pump and connected piping to facilitate repairs. Likewise, the need for surface flow sensors adds cost to the pump system. The elimination of a single sensor improves the installation cost, maintenance cost and reliability of the system.
Also, centrifugal pumps are inefficient when operating at slow speeds and/or flows, wasting electrical power. Therefore, there is a need for a method which would provide reduced flow without sacrificing overall efficiency.
Accordingly, it is an objective of the invention to provide a method for estimating the flow and pressure of a centrifugal pump without the use of down hole sensors. Another objective of the invention is to provide a method for determining pump suction pressure and/or fluid levels in the pumping system using the flow and pressure of a centrifugal pump combined with other pumping system parameters. Another objective of the invention is to provide a method for using closed loop control of suction pressure or fluid level to protect the pump from damage due to low or lost flow. Another objective of the invention is to provide a method for improving the dynamic performance of closed loop control of the pumping system. Other objectives of the invention are to provide methods for improving the operating flow range of the pump, for using estimated and measured system parameters for diagnostics and preventive maintenance, for increasing pumping system efficiency over a broad range of flow rates, and for automatically controlling the casing fluid level by adjusting the pump speed to maximize gas production from coal bed methane wells.
The apparatus of the present invention must also be of construction which is both durable and long lasting, and it should also require little or no maintenance by the user throughout its operating lifetime. In order to enhance the market appeal of the apparatus of the present invention, it should also be of inexpensive construction to thereby afford it the broadest possible market. Finally, it is also an objective that all of the aforesaid advantages and objectives be achieved without incurring any substantial relative disadvantage.